An organic light-emitting element represents a self-emissive display device, having advantages relating to thinness, light weight, wide view angle, low power consumption, high contrast, etc.
The light-emitting principle of organic light-emitting element is that holes and electrons are introduced by electrode injection, the light is generated when holes and electrons recombine in light-emitting layer and return to the ground state from the excited state. As such light-emitting elements have characteristics of being advantageously thin and being capable of emitting high brightness light at low driving voltage and emitting light in multiple colors by using different light-emitting materials, therefore, these light-emitting elements receive much concern.
Since C. W. Tang from Kodak Company reported that organic thin-film elements could emit light with high brightness, many researches have been made to applications of the organic thin-film elements. Organic thin-film light-emitting elements are now used in main screens of mobile phones and other devices, and significant progress is demonstrated on practicalization. However, there are various technical issues needs to be addressed. Particularly, realizing high efficiency and low power consumption of the element is one subject to be figured out.
Depending on the direction of the light from the organic light-emitting layer, the organic light-emitting elements are classified into two types, namely bottom-emitting organic light-emitting elements and top-emitting organic light-emitting elements. In a bottom-emitting organic light-emitting element, light is pointed to the substrate side. A reflective electrode is provided on the organic light-emitting layer, while a transparent electrode is provided at the lower section of the organic light-emitting layer. In this case, when the organic light-emitting element has an active matrix element, since the part where the thin-film transistor is formed is opaque, the light-emitting area is reduced. On the other hand, in a top-emitting organic element, the transparent electrode is formed at the upper section of the organic light-emitting layer, and the reflective electrode is formed at the lower section of the organic light-emitting layer, so the light is emitting in a direction opposite to the substrate side. Thus, the light-transmitting area is increased and the brightness is increased.
In the current technology, for improving light-emitting efficiency of the top-emitting organic light-emitting elements, one approach is to form an organic covering layer on the upper translucent metal electrode where the light form the light-emitting layer passes through, so as to modulate the optical interference distance, and control reflection of external light and extinction caused by energy movement of surface plasma (as described in Patent Documents 1 to 5).
For example, as described in Patent Document 2, an organic covering layer is formed on the upper translucent metallic electrode of the top-emitting organic light-emitting element and has a refractive index of 1.7 or more and a thickness of 600 Å, contributing to improving the light-emitting efficiency of the red and green organic light-emitting element to 1.5 times. The material of the organic covering layer uses an amine derivative or a quinolinol clathrate.
As described in Patent Document 4, materials whose energy gap is smaller than 3.2 eV have impact on the wavelength of blue light, and are not suitable for organic covering layers. The materials of the organic covering layer are amine derivatives having specific chemical structures.
Patent Document 5 describes that for the blue light-emitting element with low CIEy, the organic covering layer is made of a material having a variation of refractive index Δn>0.08 in the wavelength range of 430 nm to 460 nm. The materials used for the organic covering layer are anthracene derivatives having specific chemical structures, etc.
Patent Documents Referred:
Patent Document 1: W02001/039554;
Patent Document 2: JP2006-156390;
Patent Document 3: JP2007-103303;
Patent Document 4: JP2006-302878; and
Patent Document 5: W02011/043083.
Non-Patent Document
Journal of Applied Physics, 96, 19 (2004).
Journal of Applied Physics Letters, 76, 1650 (2000).